This invention relates to the preparation of aliphatic polyesters.
Many types of reactions can be used to synthesize polyesters. See, e.g., U.S. Pat. No. 3,912,692 disclosing a prior art use of a tin(II) catalyst to produce polyglycolide. The most important polyesterification reactions are those in which the molecular chain is built up by formation of an ester linkage at each step.
There are two primary types of such reactions: polyesterification reactions of bifunctional reactants (also called polycondensation reactions as water is deliberated) and ring-opening polymerization of cyclic esters. Both reactions occur until equilibrium is reached. Step-polyesterification and ring formation of cyclic esters are closely interrelated as they take place concurrently. This makes it difficult to produce high molecular weight polymers with sufficient purity for ordinary applications.
When producing aliphatic polyesters with a degree of polymerization of approximately 20 or below, it is normally sufficient to allow the water of reaction to be driven off at the temperature of reaction, with the assistance of stirring and the passage of a current of dry inert gas. Producing higher molecular weight polyesters, however, generally requires the use of azeotropic entrainment or reduced pressure to press the reactions forward.
Moreover, as these reactions proceed at relatively high temperatures, there is a risk of thermal degradation during the several hours that is usually required for the product to reach the desired molecular weight. For instance, polyesters of oxalic or malonic acid readily decarboxylate, 3-hydroxypropionic acids readily dehydrate, and cis-vinylene or cis-substituted alicyclic reactants readily isomerize or cross-link.
Where polyesterification is to occur by a ring opening mechanism instead of a step-polycondensation, different problems are encountered. First, these techniques require the use of expensive cyclic monomers. Moreover, the choice of cyclic monomers that may be used is limited. Cyclic monomers that transcend certain well known ring sizes will not undergo polyesterification in a commercially meaningful manner.
Another method of preparing polyesters is the reaction of a (bis)acyl chloride with a (bis)trimethylsilyl ether, producing trimethylsilyl chloride as coproduct. This reaction is described by Kricheldorf et al in both Polymer 23, 1821 (1982) and Macromol. Chem. 184, 475 (1983). This reaction is also carried out at high temperatures in either a melt or in an inert solvent. Here too, the process is relatively expensive due to the high cost of silylated compounds.
A bulk manufacturing process for producing high molecular weight aliphatic polyesters using less expensive starting materials and operating under safe and environmentally sound conditions using rather simple equipment is still needed. A resulting reduction in the cost of the aliphatic polyester is always welcome in the art. Consequently considerable research and development efforts have been made for an improved manufacturing process for these polyesters.